There is a need for high-resistance, multiple-conductor cables for use in Magnetic Resonance Imaging (MRI) procedures to monitor a patient's vital signs. The extreme magnetic fields produced by the MRI machine can interact with the patient monitoring cable producing image artifacts which corrupt the MRI image. Image artifacts can be minimized or eliminated by using a cable with a high enough resistance so that it is not affected by induction currents generated by the magnetic field.
Current manufacturing methods are capable of producing such cables using round wire and wire harness manufacturing techniques but these cable assemblies are relatively expensive to produce. These current techniques also produce cables which contain too much variation in the resistance of the wires within each round wire cable. This invention significantly reduces the manufacturing cost for a high-resistance multiple conductor cable by adapting it to the manufacturing techniques used for producing inexpensive printed thick film (PTF) circuits.
High-resistance conductors can be created by screen printing or other similar coating processes to apply a conductive carbon ink or other conductive polymer to a flat, electrically insulating substrate such as polyester or polyimide film to create a high-resistance PTF circuit, but not with the precision possible with the techniques of the present invention. In this application, there is a need for the conductors of the cable to be closely matched in resistance to one another, generally within +/−5%.
In this application, the cable lengths are typically in excess of six feet and this cannot be consistently achieved with current manufacturing techniques. Standard conductive ink application processes do not provide tight enough control over the conductive ink variables such as thickness, width, density, and the like to produce consistent high-resistance conductors that are matched in resistance to within +/−5%.
For example, with current screen printing methods, the screen needed to produce long cables would be expensive. Another limitation of current screen printing techniques is the ability to lay down multiple layers of ink in order to consistently get the desired thickness. This is in part because each layer needs to be cured before another layer can be applied. The curing process causes shrinkage and variations in the resulting resistance of the conductor. Also, it is difficult to align the previously screened image to subsequent images to create accurate multilayer deposits when printing fine lines. Additionally, the screens have limitations on the size of the mesh available, which also limits the thicknesses of the layers that are possible with current screen printing techniques.
Current printing methods can typically only be used to produce conductors matched in resistance within +/−25% when using the current state of the art practices. Other current printing methods such as pad printing and roll printing also have similar limitations on the ability to control the thickness, width, density, and the like of the conductive ink in order to produce consistent, high-resistance conductors that are matched in resistance within +/−5%.
Existing methods for adjusting the resistance of a printed carbon ink or conductive polymer trace are also limited. The existing methods use a mechanical or abrasive process, such as abrasive media blasting, or laser ablation to remove material to reduce the width and/or increase the length of the conductive path, thereby increasing the resistance. This process can be time consuming, expensive, and inexact. See FIG. 1.